Re: mlxsw: spectrum: PTP: Support timestamping on Spectrum-1 - potential null ptr dereference

[Colin Ian King]

On 02/07/2019 16:51, Petr Machata wrote:
> 
> Colin Ian King <colin.king@xxxxxxxxxxxxx> writes:
> 
>> Hi,
>>
>> Static analysis with Coverity on today's linux-next has found a
>> potential null pointer dereference bug with the following commit:
>>
>> commit d92e4e6e33c8b19635be70fb8935b627d2e4f8fe
>> Author: Petr Machata <petrm@xxxxxxxxxxxx>
>> Date:   Sun Jun 30 09:04:56 2019 +0300
>>
>>     mlxsw: spectrum: PTP: Support timestamping on Spectrum-1
>>
>>
>> In function: mlxsw_sp1_ptp_packet_finish the offending code is as follows:
>>
>>        /* Between capturing the packet and finishing it, there is a
>> window of
>>         * opportunity for the originating port to go away (e.g. due to a
>>         * split). Also make sure the SKB device reference is still valid.
>>         */
>>        mlxsw_sp_port = mlxsw_sp->ports[local_port];
>>        if (!mlxsw_sp_port && (!skb->dev || skb->dev == mlxsw_sp_port->dev)) {
>>                dev_kfree_skb_any(skb);
>>                return;
>>        }
>>
>> If mlxsw_sp_port is null and skb->dev is not-null then the comparison
>> "skb->dev == mlxsw_sp_port->dev" ends up with a null pointer dereference.
>>
>> I think the if statement should be:
>>
>> if (mlxsw_sp_port && (!skb->dev || skb->dev == mlxsw_sp_port->dev))
>>
>> ..but I'm not 100% sure as I may be missing something a bit more subtle
>> here.
> 
> Yes, that line is wrong. It's missing a pair of parens, it should be:
> 
>         if (!(mlxsw_sp_port && (!skb->dev || skb->dev == mlxsw_sp_port->dev))) {
> 
> I.e. I need a port && I need the skb->dev to still refer to that port
> (or else be NULL). If that doesn't hold, bail out.

Ah, that makes sense.

> 
> Thanks for the report, I'll spin a fix!
> 
OK, thanks!